Leave the stars on screen on a touch device

**Hover is not something a finger does, but Slint reports it anyway.**
`has-hover` goes true for any pointer event carrying a position, a touch
press included, and false again on the `Exit` that follows the release.

So the rating strip did appear on a tablet — for exactly the length of a
tap. It flashed on under the finger, vanished as it lifted, and the tap
carried on through to the cell and opened the photograph. An unjudged
frame could not be rated from the grid at all. The previous fix stopped
the strip disappearing when a *pointer* moved onto it; this is the same
symptom with a different cause, and hover was the wrong signal in the
first place.

The strip now stands open where the session is a touch one. That is
seeded from the platform rather than inferred, because inference needs a
press to reach a cell and a quick flick never delivers one — the Flickable
claims the gesture before the delay it would forward after — and a
control that only appears once the finger is down has appeared too late
to aim at. The grid still latches on the first non-zero touch id it sees,
which is what covers a touchscreen on the desktop.

One-way on purpose: a tablet with a mouse plugged in keeps the strips
once touched, which is the harmless direction to be wrong in. The
alternative is chrome that comes and goes as the user changes hands.

ui/dr-ui/ui/app.slint

@@ -507,6 +507,10 @@ export component AppWindow inherits Window {
     in-out property <int> library-columns: 1;
     in property <bool> library-syncing: false;
     in-out property <length> library-cell-size: 180px;
+    /// Whether this session is a touch one, which decides if the rating strip
+    /// waits to be hovered or stands open. Seeded from the platform by Rust
+    /// and latched by the grid on the first finger it sees.
+    in-out property <bool> library-touched: false;
     callback library-zoom-cells(int);
     /// A pinch step on the grid: the ratio since the last update.
     callback library-pinch-cells(float);
@@ -1133,6 +1137,7 @@ in property <bool> panel-visible: true;
             timeline-zoom(d) => { root.library-timeline-zoom(d); }
             syncing: root.library-syncing;
             requested-cell-size: root.library-cell-size;
+            touched <=> root.library-touched;
             zoom-cells(d) => { root.library-zoom-cells(d); }
             pinch-cells(r) => { root.library-pinch-cells(r); }
             pinch-started() => { root.library-pinch-started(); }